Electric Lime vs Sunny Yellow

How close are they?

ΔE 18.5, loose: different enough that you should pick on purpose. Sunny Yellow is noticeably lighter, more muted than Electric Lime.

Can I use Sunny Yellow instead of Electric Lime?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Sunny Yellow is noticeably lighter, more muted than Electric Lime.

Can I use Electric Lime instead of Sunny Yellow?

The same difference applies the other way round: Electric Lime is noticeably darker, more saturated.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.